Inside the Red Cube

[DanH]

A while back my buddy Tom asked if I'd like a red cube fuel flow transducer for a bit of "forensic examination", as he had recently replaced one which wasn't working right.

Of course I said "Oh yeahhhh"

The manufacturer has designed the device so it cannot be dismantled for overhaul, at least not without a lot of special effort. First, the allen screws on the cover plates are installed with a thread locker, and being long, small diameter screws, most just twist off before the locker will let go. Second, the electronics are potted. Third, one wire is routed and potted in such a way that removing a cover plate breaks the wire, and it can't be repaired. In this case I didn't care about repair, so brute force methods were employed. Here we go:

There are cover plates on both side of the cube. One side has two stacked plates, and the other has a single plate. The single plate side just covers an potted electronics bay. I did not investigate the electronics.



On the other side, removing the topmost plate exposes the outer face of this machined assembly. The potted groove contain a wire:



The inside face contains a jewel bearing in the center, and an LED (or a receiver, don't know which, see below). You can also see the broken stub of the previously mentioned wire. Removing this assembly from the cube body breaks the wire, which was potted in both the cube and the plate after they were assembled. Clever.



Inside the main body we find another bearing, and a matching LED (or receiver). You can see the (broken) potted wire at about 5 o'clock:



There are two ports for fuel flow, arranged tangent to the circle:



Suspended in the circular cavity between the two bearings is a plastic rotor with 20 teeth and four impellers:



The "teeth" break the light beam between the LED and receiver, 68,000 times per gallon, as the rotor spins in the flow:



When the cube is installed between a boost pump and the engine driven pump, most builders report an excessive fuel flow indication with the boost pump running. I don't think any of us fully understand the flow mechanics with precision, but looking at the rotor, it's not hard to see how a little bit of pulsing in the flow might result in very small reversals of the wheel rotation, which would cause extra counts.

That's all folks.

[rv6ejguy]

Interesting to see inside one of these. Thanks Dan.

For SDS EFI users, we offer a $100 option to output a similar FF signal without any Red Cubes needed. I know several people have asked whether they need these our not with our system.

[Lars]

Originally Posted by DanH

When the cube is installed between a boost pump and the engine driven pump, most builders report an excessive fuel flow indication with the boost pump running. I don't think any of us fully understand the flow mechanics with precision, but looking at the rotor, it's not hard to see how a little bit of pulsing in the flow might result in very small reversals of the wheel rotation, which would cause extra counts.

That's all folks.

Which makes sense. Without taking a stab at the math, guessing that the (rotational) inertia of the wheel is substantially less than the (linear) inertia of the fuel passing through the device. Small changes in the latter would affect the former.

[lrgabriel]

Wow, Dan, thanks for posting this!

[Carl Froehlich]

Way cool. Thanks for posting.

Carl

[SmilingJack]

Are you cousins with "Macgyver"

Neat stuff! Now I know I will never touch that thing!

[Chkaharyer99]

Thanks for showing the magic inside the Red Cube. My cube is between my Airflow Performance fuel pump and the mechanical engine driven pump. Your illustration/tutorial helps me to better understand why I get a higher fuel flow indication with the electric pump on. I had no idea the Red Cube was a electro/mechanical device.

Thanks Dan.

[TS Flightlines]

Easier to understand how things work when you take them apart!
Tom

[rv7boy]

Dan, you are amazing!

I offer a question and then a comment:

1) Where did you get the factor of 68,000 counts per gallon for the unit? I assume it's from the manufacturer but where?

2) In the interest of accuracy, and I know you like to be accurate, I offer the comment that the term "impeller" is usually used to describe a part that transfers energy TO the fluid. A better term for the part that receives energy FROM the fluid might be "vane" or "blade." I know it's a nit.

Thanks for doing the forensics. I think you should offer your services to the NTSB as a field resource for aviation accident investigations.

P.S. My ancient educational methods yield a rotational speed of the little rotor of around 57 RPM for a steady state flow rate of 10 GPH through the unit. That's based on your calibration factor of 68,000 counts per gallon.

[darata]

That is SO out of warranty!!